1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish, and more particularly to a polishing apparatus having a cleaning device for cleaning a top ring for supporting a workpiece and/or the workpiece which has been polished.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus, and such a process is called Chemical Mechanical polishing.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated.
After, for example, one or more semiconductor wafers have been polished, the polishing cloth is processed to recover its original polishing capability. Various processes have been and are being developed for restoring the polishing cloth, and are collectively called "dressing". The polishing cloth is dressed by a dressing tool in order to enable the polishing apparatus to perform a good polishing function at all times without undesired degradation of polishing performance.
The semiconductor wafer which has been polished carries abrasive liquid and ground-off particles attached thereto. After a semiconductor wafer has been polished, ground-off particles and the abrasive liquid are also attached to the top ring because the semiconductor wafer has been held by the top ring. Therefore, the polished semiconductor wafer and the top ring have to be cleaned to remove the foreign matter including abrasive liquid and ground-off particles therefrom.
It has been customary to install a robot to transfer a semiconductor wafer therefrom to the top ring before it is polished, and to transfer the semiconductor wafer from the top ring thereto after it is polished. That is, the semiconductor wafer is transferred directly between the top ring and the hand of the robot which is associated with the polishing apparatus.
Consequently, the top ring which has been used for polishing the semiconductor wafer, and the semiconductor wafer which has been polished are cleaned as follows:
(1) First, the polished surface of the semiconductor wafer which is held by the top ring is cleaned by a cleaning liquid ejected from a cleaning nozzle. PA1 (2) Then, the semiconductor wafer is transferred from the top ring to the hand of the robot. PA1 (3) The lower surface (holding surface) of the top ring which has held the semiconductor wafer is cleaned by a cleaning liquid ejected from the cleaning nozzle. PA1 (4) Finally, a next semiconductor wafer to be polished is transferred from the hand of the robot to the top ring.
However, since the above conventional cleaning process involves the two cleaning steps (1) and (3), it takes a relatively long period of time to clean the semiconductor wafer and the top ring. As a consequence, the conventional polishing apparatus cannot polish the semiconductor wafers with increased efficiency.
Further, in the step (3) of the above steps, since the lower surface of the top ring is not cleaned immediately after the semiconductor wafer is polished, the abrasive liquid applied to the lower surface of the top ring tends to solidify. The cleaning liquid and ground-off particles are scattered over the polishing cloth, thus tending to alter the composition of the abrasive Liquid for polishing a workpiece, with the result that the polishing performance of the polishing apparatus is varied. Bristles of a brush are liable to drop onto the polishing cloth, and give damage to a semiconductor wafer which is being polished by the polishing cloth.
Furthermore, the dressing tool suffers similar problems because it is not cleaned immediately after it has dressed the polishing cloth.